Hmmm.... That concept had crossed my mind, but I couldn't think how linear it was. If you imagine I had a very big diameter mandrel, then the 8mm rod would spring bach to a straight rod, whereas with a very small diameter, the spring back will be very small, and when released, would end up almost hugging the mandrel, so the ratio is not linear, but I suppose within a limited range it could be approximated to linear. The problem is I can't think what would be a reasonable range that I could assume the ratio to be approximately linear. I hope you can see my point. Thanks for your input.